CIRCULATION OF THE CCELOMIC FLUID. 185 



directions occurs because the uterus does not completely fill the 

 anterior end of the ccelom and consequently when it constricts 

 it allows some fluid to move anteriorly. As a result, there is 

 always a certain amount of fluid left at this end even after the 

 last antero-posterior peristaltic wave has passed on. Another 

 point of difference in the processes at the two ends is the fact 

 that at the posterior end there is an instant's pause after the 

 arrival of the fluid before it is sent back, while at the anterior 

 end there is no pause, the peristalsis being immediately reversed. 

 In view of this pause the posterior end can be considered as 

 marking the beginning and the ending of the circulatory cycle. 

 Seven cycles of the primary wave occur in two minutes. This 

 rate was maintained by each of two worms in which the process 

 was timed. 



Frequently all of the secondary waves moving in a given 

 direction have not reached the end toward which they are 

 travelling before a primary wave starts toward them from that 

 end. When this occurs the secondary wave keeps on moving 

 until it meets the primary wave which then absorbs it and con- 

 tinues onward. When, as in the case of the posterior end, there 

 is a momentary pause in the peristalsis of the uterus, the fluid 

 does not necessarily come to rest throughout the entire coelom, 

 but secondary waves which may be moving keep on their course 

 until they have reached the posterior end or until the primary 

 wave from that end has met them. 



Just why these secondary waves continue to move in this 

 manner my observations did not make quite clear. The condi- 

 tion might be brought about by peristalsis from one end con- 

 tinuing until it met and was overcome by that originating at 

 the opposite end. I am not prepared to say that this is the 

 case, especially since, postero-anterior peristalsis begins at the 

 anterior end almost synchronously with that at the posterior end. 

 Possibly the impetus received from the peristalsis is sufficient 

 to keep the secondary waves moving for a time even after 

 peristaltic action has ceased. 



As a wave moves along, a certain amount of the fluid com- 

 posing it trails behind and comes to rest. This slipping back 

 occurs because the uterus does not completely fill the ccelom. 



